Patent Document 1 (shown below) discloses a control system for an internal combustion engine, wherein a residual gas ratio (an internal exhaust gas recirculation ratio), which is a residual ratio of burnt gases remaining in the combustion chamber after combustion, is calculated, and the ignition timing is controlled according to the residual gas ratio. According to this control system, the residual gas ratio is calculated based on the engine rotational speed, the valve overlap amount (an overlapped period of the valve opening periods corresponding to the intake valve and the exhaust valve), the intake pressure, the exhaust gas temperature, and the intake air amount.
Further, a known control system for an internal combustion engine having an exhaust gas recirculation mechanism, uses a method for calculating an exhaust gas recirculation ratio using a map for calculating the exhaust gas recirculation ratio (the external exhaust gas recirculation ratio), the map being set according to an opening of the exhaust gas recirculation control valve.